In a multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) system, resource allocation is implemented in a subchannel unit consisting of a plurality of subcarriers. That is, a plurality of users divide and share the whole subcarrier, thereby being capable of obtaining a multi-user diversity gain in a frequency domain. In a broadband wireless communication system, all cells reuse the same frequency, and apply an adaptive modulation and coding (AMC) scheme according to the received signal strength and interference between adjacent cells, thereby maximizing a throughput.
FIGS. 1A and 1B illustrate examples of cell frequency arrangement and frequency reuse. As shown in FIGS. 1A and 1B, the same frequency is reused at a distance and this is called “frequency reuse”. A frequency reuse factor ‘K’ of importance to a mobile communication system is a rate representing that the same frequency is reused every how many cells. As the frequency reuse factor ‘K’ increases, a distance between cells using the same frequency increases, thus decreasing the influence of interference caused by the use of the same frequency.
In FIG. 1A where a frequency reuse factor ‘K’ is equal to three (3), three (3) frequencies are reused at a specific distance. In FIG. 1B where a frequency reuse factor ‘K’ is equal to seven (7), seven (7) frequencies are reused at a specific distance.
A system having a frequency reuse factor of one (1) suffers serious inter-cell interference at a cell or sector boundary, thus causing inevitable throughput reduction and also encountering service outage circumstances. A fractional frequency reuse (FFR) technique is a method for improving performance at a cell or sector boundary when a frequency reuse factor is equal to one (1). The FFR technique reduces co-channel interference between adjacent cells by orthogonally dividing the whole subcarrier into a plurality of subbands, properly arranging the subbands, and avoiding the use of a part of the subbands in each cell. In other words, the FFR technique has been introduced to solve a problem that a low carrier to interference and noise ratio (CINR) leads to a reduction of performance of a mobile station (MS) located in a boundary area of each cell because all cells use the same frequency resource.
The FFR technique uses the fact that mobile stations (MSs) located in a cell center area and a cell boundary area are differently influenced by interference from an adjacent cell. That is, when an MS is located close to a serving base station (BS) in a cell center area, the MS is slightly influenced by path loss and is relatively slightly reduced in signal component reception sensitivity. However, the MS is located relatively far away from a neighboring interfering BS, thus being greatly influenced by path loss. As a result, the influence of co-channel interference (CCI) decreases. On the other hand, when an MS is located in a cell boundary area and is at a similar distance from both a serving BS and an interfering BS, the MS receives both a signal component and an interference component at a similar reception sensitivity. As a result, the influence of CCI increases. Accordingly, the FFR technique allows an MS located in a cell center area to use a resource whose frequency reuse factor is equal to one (1) and allows an MS located in a cell boundary area to use a resource whose frequency reuse factor is more than one (1), thus ensuring a reception performance to an MS located in a boundary to a certain extent.
In the FFR technique, a “common zone” is called a zone of a frequency reuse factor of one (1) used by an MS located in a cell center area and a “protected zone” is called a zone of a frequency reuse factor of more than one (1) used by an MS located in a cell boundary area.
The protected zone serves as a restricted zone for MSs located in other cell boundary areas. That is, the restricted zone is basically an empty zone unused by each BS to suppress co-channel interference. Each serving BS uses only the minimum transmit power to limit its interference influence on a neighboring BS only up to an allowable level, thus being capable of allocating a restricted zone to an MS located close to the serving BS for use.
As described above, a BS may determine the maximum transmit power for the restricted zone in addition to the common zone, determine a modulation and coding scheme (MCS) level, and perform resource allocation. This causes a problem of generating an overhead because a channel quality indicator (CQI) feedback may be implemented to determine the maximum transmit power for the restricted zone and a CQI feedback may be implemented to determine the maximum transmit power for the common zone.